1. Field of the Invention
The present invention relates to a position measurement system and a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to accurately transfer the pattern onto the target portion of the substrate, the relative position of the pattern and the target portion should be known. Therefore, the lithographic apparatus is in general equipped with one or more measurement systems to determine the position of, e.g., the substrate or the patterning device. Examples of such measurement systems are interferometer systems or encoder systems. Both systems can be designated as incremental systems. Using such a position measurement system, the position of an object can be determined relative to a chosen reference as an integer number of increments (or periods) of a predefined length. Using an interferometer, this increment may, e.g., correspond to a quarter of the wavelength of the interferometer laser. In case of an encoder system, the increment may, e.g., correspond to a quarter of the period of the encoder grating.
In order to improve the resolution of such an incremental measurement system, methods are developed to provide an interpolation within one increment (or period).
Such a position measurement system usually comprises an incremental measurement unit comprising a first part comprising a sensor and a second part co-operating with the sensor of the first part. In case of an interferometer system, the second part may comprise a mirror for reflecting a beam originating from the interferometer laser to the sensor. In case of an encoder system, the second part may comprise a one- or two-dimensional grating co-operating with the sensor (in this case, the sensor usually comprises an encoder head). Because of the limited size of, e.g., the mirror or the grating, the operating range of the measurement system is limited. In order to increase the operating range, the measurement system can be equipped with more that one sensor arranged on different locations along a required operating range ensuring that the position measurement can be performed over the entire required operating range. In such a multi-sensor measurement system, problems may arise during the transition of the position measurement by a first sensor to the position measurement by a second sensor. Conventionally, one (or more) measurement values of the first sensor are used to initialize the second sensor during the transition (such initialization may be required because the initial measurement by the second sensor may not be related to a reference). Because this initialization is based upon measurements of both the first sensor and the second sensor, measurements that may contain a measurement error, this initialization may result in an increased measurement error for the second sensor. During a next transition (either a transition of a measurement by the second sensor to a measurement by a third sensor or a transition of a measurement by the second sensor to a measurement by the first sensor) a further increase in the measurement error may occur. As such, the accuracy of a multi-sensor measurement system used in a conventional way may deteriorate due to transition from a measurement by one sensor to a measurement by another sensor.